In bipolar transistors, it is necessary to make contact with and to connect to the region of the intrinsic base with as low an impedance as possible in order to achieve very high cut-off frequencies and to reduce the RF noise. This is achieved particularly advantageously by a base connection which is as short as possible, is preferably self-adjusting, and thus has a low impedance.
In the case of so-called “single-poly bipolar technologies”, this base connection region is normally defined lithographically, that is to say not in a self-adjusting manner. By way of example, the lithographically defined emitter plug is used as an implantation hard mask for the base contact. Since the doping of the intrinsic base defines the current gain of the bipolar transistor, it cannot be chosen to be indefinitely high. If the base connection and/or the base connection region which directly touches the intrinsic base are/is not formed by a dedicated, heavily-doped polysilicon layer, which allows contact to be made with the intrinsic base in a self-adjusting manner, the base connection is normally heavily doped by ion implantation by means of, for example, a photoresist mask. In this case, the minimum distance between the heavily doped component of the base connection and the active transistor area, that is to say the intrinsic base, is governed by the lithography tolerances. For this reason, the base connection in “single-poly bipolar transistors” such as these has a relatively high impedance, since the heavily doped connection region is connected to the intrinsic base through this relatively long region, which is doped only with the low dopant concentration of the intrinsic base.
Conventional bipolar transistors thus often use two polysilicon layers to provide a contact, with one being intended for the emitter contact, and one for the base contact. In this case, an emitter window, which defines an actual contact area between the emitter and the intrinsic base, is etched by the heavily doped base connection polysilicon. The base connection is thus automatically located in a self-adjusting manner alongside the emitter window, in which the intrinsic base is defined. In this variant, the low-impedance base connection is obtained by outward diffusion from the base connection polysilicon to the intrinsic base.
In a conventional single-poly bipolar transistor dedicated polysilicon deposition or a dedicated polysilicon layer is used only for the emitter contact, the base connection region is normally defined lithographically, and is heavily doped by means of implantations. Since this process is in consequence not self-adjusting, it thus includes all of the lithography fluctuations and tolerances.